The plant hormone auxin is involved in virtually all aspects of plant growth and development. Remarkably, auxin exerts its affects over a broad concentration range. At the cellular level, auxin acts to regulate cell expansion, cell division, ad cell fate. One of the most interesting and challenging questions in plant biology is how this simple molecule elicits such a complex set of responses. We have focused on auxin regulation of transcription. Over the years, we have shown that auxin acts by stimulating the degradation of a family of transcriptional repressors called the Aux/IAA proteins, through the action of the ubiquitin protein ligase SCFTIR1/AFB. During the last grant period we demonstrated that auxin is perceived by a co-receptor consisting of TIR1 or an AFB protein plus an Aux/IAA protein. Importantly, different co-receptor pairs differ in their affinity for auxin. The existence of high nd low affinity co-receptors dramatically enhances the dynamic range of the hormone and may contribute to the complexity of auxin responses. In addition, our recent work led to the discovery that the chaperone HSP90 and co-chaperone SGT1 are involved in TIR1 function, and that auxin response in the hypocotyl or seedling stem, is facilitated by a positive feedback loop that involves a family of proteins called the PREs. The long-term goals of this proposal are to determine the molecular basis of auxin signaling. Our specific aims are to 1) test the biological significance of the auxin co-receptor model, 2) to investigate the role of HSP90 and SGT1 in SCFTIR1 function, 3) to characterize the PRE positive feedback loop in auxin-regulated hypocotyl elongation. These studies address a number of key issues in cellular regulation and will have important implications for human health. The ubiquitin pathway and the SCFs in particular are involved in diverse disease processes including numerous cancers. Because SCFTIR1 is one of the best- characterized E3 complexes in any organism, this work provides a unique opportunity to advance our understanding of this critical aspect of human disease.